A) Field of the Invention
The present invention relates to a semiconductor device and more particularly to a semiconductor quantum dot device using semiconductor quantum dots.
B) Description of the Related Art
Various semiconductor devices are known which use the quantum effect. High electron mobility transistors (HEMTs) and emission devices using a quantum well (QW) and a multiquantum well (MQW) are well known as semiconductor devices using one-dimensional quantization in a thickness direction.
Known types of quantization other than one-dimension include quantum wires using two-dimensional quantization and quantum dots (or quantum boxes) using three-dimensional quantization. Various emission devices using quantum dots have recently been developed.
Fabrication techniques for self-grown quantum dots using lattice mismatched material in a Stranski-Krastanov growth mode (S-K mode) have been researched diligently starting in the mid-1990s. As material having a large lattice mismatch is grown on a substrate, a wetting-layer is formed initially. As the layer becomes thick, accumulated strains become large so that three-dimensional growth occurs to form islands such as semi-spheres. These islands can be used as quantum dots. There are recent reports on devices such as a semiconductor laser using such self-grown quantum dots. The possibility of quantum dot devices is becoming real.
In fabricating a quantum dot laser for an optical communication light source, it is desired to form quantum dots having an emission wavelength of 1.3 μm (emission energy of 0.95 eV) or an emission wavelength of 1.55 μm (emission energy of 0.8 eV).
For example, there is a report on a semiconductor quantum dot laser which is fabricated by forming InGaAs dots in an active layer on a GaAs substrate and it can demonstrate continuous oscillation in a 1.3 μm band at a room temperature (IEEE PHOTONICS TECHNOLOGY LETTERS, Vol. 11, 1205, 1999).
There is a report on InAs quantum dots in a 1.55 μm band formed on an InGaAlAs spacer layer on an InP (311) B substrate (J. Crystal Growth 245, 31, (2002)). Lattice mismatches of InAs and InGaAlAs with the InP substrate are opposite in polarity and the influences thereof cancel out each other. Emission at a wavelength of 1.58 μm is observed by photoluminescence.
The present inventor and his colleague have proposed the structure that an lnGaAsP barrier layer and an InAsSb quantum dot layer are alternately stacked on an InP substrate (JP-A-2005-136267). Mixed crystal InAsSb has large band gap bowing: a band gap of 0.35 eV at an As composition of 1.0 (InAs) and a minimum band gap of 0.1 eV at an As composition of about 0.6, and can change the band gap in a range from 0.1 to 0.35 eV by changing the composition. Since a lattice constant is larger than that of InAs, the size of an InAsSb quantum dot is smaller than an InAs quantum dot. The mixed crystal InAsSb is a material having the possibility of providing quantum dots having a large quantum effect.